346 



COMPARATIVE ANATOMY 



CHAP. 



the imbrication of the ambulacra is adoral, i.e. the oral edge of each plate overlaps 

 the apical edge of the next in order below it, whereas, in the interambulacra, the 

 imbrication is apical. Lateral imbrication also occasionally occurs. 

 Slight imbrication is also found in certain Spatangoida. 



(/) Special Modifications of the Ambulacra. 



In all Echinoidea, in which the mouth remains at the centre of the oral surface, 

 the five ambulacra are alike in length, breadth, and in the arrangement of their 



in 



F,o. 306.-Sy*tem of plates of a Clypeastroid (Encope Valenciennesi, Agass.), spread out 



(after Loven). 



P mi " el ' Ce f S ' etc :, The y 'y y length when the apieal system, towards 



from thf J" g f7 . PeriSt me aCTOSS the ambitus ' the y c n ' e ' * shif ted 

 -he centre of the ap.cal hemisphere to a somewhat anterior (less frequently 

 1-ostenor) pos.bon. If the test of such an Echinoid, in which the ambulacra 



